The present invention relates to an absorbent polymer and a process for making an absorbent polymer. More specifically, the present invention relates to a networked polymer/clay alloy useful for example, without limitation, in containment applications as landfill liners or covers, reservoir liners, underground storage tank liners, secondary containment liners, and man-made bodies of water, or personal care absorbent articles, including diapers, training pants, feminine hygiene products such as sanitary napkins, incontinence devices and the like.
Super absorbent polymers (xe2x80x9cSAPsxe2x80x9d) have been produced since the 1970s for use in a variety of products including, amongst others, hygiene products, such as disposable diapers, training pants, feminine hygiene products and incontinence devices, agricultural and horticultural products and industrial and environmental absorbents. SAPs are primarily utilized to increase or enhance the product""s water-absorbency.
SAPs are produced from a variety of components by a variety of processes. For example, SAPs are often made from monomers such as acrylamide, acrylic acid and acrylate, which are particularly suitable for application in hygiene products.
For example, U.S. Pat. No. 4,032,701 (Hughes, Jun. 28, 1977) describes a process for producing polyacrylamide in dry and solid form using a polymerization catalyst selected from a group consisting of an alkali metal and ammonium sulfite, bisulfite and persulfate. U.S. Pat. No. 4,178,221 (Boutin et al, Dec. 11, 1979) describes a process for producing water-soluble acrylic polymers using a sequential photopolymerization technique. Photopolymerization promoters are incorporated in the monomer solution to facilitate polymerization. U.S. Pat. No. 4,283,517 (Perricone et al, Aug. 11, 1981) produces polyacrylamide using a quatemary ammonium salt as a cross-linking agent, while U.S. Pat. No. 4,295,987 (Parks, Oct. 20, 1981) uses two cross-linking agents to produce a cross-linked sodium polyacrylate absorbent.
Further examples of the production of SAPs providing improved properties are provided by U.S. Pat. No. 4,731,067 (Le-Khac, Mar. 15, 1988), U.S. Pat. No. 5,185,409 (Sortwell, Feb. 9, 1993), U.S. Pat. No. 5,145,906 (Chambers et al, Sep. 8, 1992), U.S. Pat. No. 5,462,972 (Smith et al, Oct. 31, 1995), U.S. Pat. No. 5,672,633 (Brehm et al, Sep. 30, 1997) and U.S. Pat. No. 5,858,535 (Wang et al, Jan. 12, 1999).
The SAP produced by each of the above-noted patents is manufactured from a chemical monomer to produce a synthetic polymer. Such chemical monomers tend to be relatively expensive and therefore, the use of the SAP produced therefrom tends to be limited to applications requiring a relatively small amount of SAP. For example, SAP made from chemical monomers tends to be too expensive for use in environmental applications given the large volumes that are typically required. The most significant expense in producing SAP is the cost of the chemical monomer. In addition, these synthetic polymers may be subject to chemical, electromagnetic (radiation) and biological (bacterial) degradation when placed in the surface environment.
Alternately, swelling clays may be used to provide water-absorbency to a product. With respect to cost, the cost of swelling clays tends to be minimal compared to that of the chemical monomers described above. In addition, swelling clays are relatively stable compared to chemical monomers and are not as subject to degradation. However, swelling clays have a water absorption capacity significantly less than that of SAP.
Thus, in order to reduce the cost of producing SAP and address the problems associated with using SAP in some applications, the polymers may be physically mixed into swelling clays to form a composite. Alternately, the monomers may be intercalated in the swelling clays and polymerized into a nanocomposite. In either event, the incorporation of the swelling clays into the SAP reduces the total cost of the SAP and enhances its resistance to chemical, electromagnetic and biological degradation, while still providing an improved water absorption capacity as compared to that of the swelling clays alone.
As indicated one technique for producing the improved SAP is to physically mix the polymer or otherwise intercalate or combine the polymer with the swelling clay to produce a water absorbent composite. For example, U.S. Pat. No. 4,418,163 (Murakami et al, Nov. 29, 1983) describes a method of making a water absorbing composite that is comprised of an inorganic powder and an absorbent resin covering the surfaces of the individual particles of the inorganic powder. The inorganic powder is white carbon, synthetic silicate white carbon, basic magnesium carbonate, ultrafine magnesium silicate, light and heavy calcium carbonate, soft and hard clay, talc, vermiculite, pearlite, barium sulfate (baryte) or mica. Thus, this patent describes a process for coating an inorganic powder with a polymer. Similar processes are described in U.S. Pat. No. 4,889,885 (Usuki et al, Dec. 26, 1989) and U.S. Pat. No. 5,352,287 (Wason et al, Oct. 4, 1994).
An alternative technique for producing the improved SAP is to polymerize an intercalated monomer. For example, Blumstein, R. et al (Applied Polymer Symposium 25: 81-88; 1974) prepares a clay-polymer complex with monolayers between the structural layers of clay minerals, namely montmorillonite clay. Specifically, clay-monomer complexes, of nearly monolayer coverage, are polymerized through free radical initiation with xcex3-ray irradiation to produce clay-polymer complexes. Blumstein, A. (Journal of Polymer Science: Part A, 3:2653-2661; 1995) similarly describes the polymerization of monolayers of an acrylic monomer adsorbed on the surface of the clay, namely montmorillonite, initiated with xcex3-ray irradiation or by free radical catalysts.
Similarly, Chinese Patent No. 85-1-02156-A (Jan. 14, 1987) describes a method of preparing a bentonite-acrylamide based SAP using cobalt-60 xcex3-ray irradiation. Specifically, the Chinese patent uses xcex3-ray irradiation to initialize polymerization. As well, Nagae H. et al (Kobunshi Ronbun 47:8:631-638; 1990) describes the preparation of complex composite films by adding acrylamide and water to montmorillonite and polymerizing the product using xcex3-ray irradiation. Thus, as with Blumstein, each of these processes requires an irradiation source for polymerization.
Further, Ogawa M. et al, (Clay Science 7:243-251; 1989) describes the preparation of montmorillonite-polyacrylamide intercalation compounds by polymerizing the intercalated acrylamide monomers in the interlayer region of the montmorillonite using a free radical initiator. The polymerization is performed using a relatively complex process involving the use of an organic solvent, namely n-heptane. First, the montmorillonite is intercalated into an acrylamide aqueous solution. The product is then dried and washed with an organic solvent, namely CCl4 or n-heptane, to remove excess acrylamide. Finally, the intercalated acrylamide is polymerized by heating the intercalation compounds with benzoyl peroxide as an initiator in n-heptane.
Kato, C. et al (Clays and Clay Minerals 29:4:294-298; 1981) describes the polymerization of intercalation compounds of styrene and ammonium montmorillonite. Specifically, clay suspensions, namely montmorillonite, are mixed with ammonium solutions. After washing and drying the resulting product, the dried organoammonium-montmorillonites are immersed in styrene monomer. The resulting stearyltrimethylammonium-montmorillonite is dried and polymerized using benzoyl peroxide as an initiator.
It would be desirable to produce an absorbent material having intimately integrated components that do not disperse and/or migrate from the product.
According to the invention, there is provided a process for producing a networked polymer/clay alloy, comprising the steps of: (a) preparing a monomer/clay mixture by mixing at least a monomer, clay particles, a cross-linking agent, and a mixing fluid in a vessel; (b) exposing the monomer/clay mixture to a thermal initiator means comprising at least one thermal initiator and thermal energy, wherein exposing the monomer/clay mixture to the thermal initiator means comprises: (i) mixing, in any order, the at least one thermal initiator with at least the monomer, clay particles, the cross-linking agent and the mixing fluid so that a monomer/clay mixture is exposed to the thermal initiator; (ii) exposing the monomer/clay mixture to a thermal energy source at a temperature in a range from about 40xc2x0 C. to about 95xc2x0 C.; and (c) polymerizing the monomer/clay mixture so that a networked polymer/clay alloy is formed.